1. Field of the Invention
The invention relates to a circuit for recovering the carrier of a digitally modulated wave having a phase symmetry 2.pi./M, where M is the symmetry order. The circuit includes means for fast automatic acquisition comprising a voltage-controlled oscillator having an output supplying said carrier and a control input to which an error signal .epsilon.(.phi.) is applied in order to change the oscillator phase and to adjust it to the phase of said digitally modulated wave. Said wave is introduced:
in a first channel comprising the series arrangement of: PA0 in a second channel comprising the series arrangement of:
a demodulator receiving said modulated wave and the oscillator output signal, and producing an in-phase demodulated signal X.sub.1, PA1 a low-pass filter acting on the demodulated signal X.sub.1 and producing a filtered signal X, PA1 means for reconstructing a signal X and for determining the error e.sub.x =X-X between the signals X and X, PA1 a phase-shifter for shifting the phase of the oscillator through 90.degree., PA1 a demodulator receiving said modulated wave and the phase shifter output signal and producing a quadrature-phase demodulated signal Y.sub.1, PA1 a low-pass filter acting on the demodulated signal Y.sub.1 and producing a filtered signal Y, PA1 means for reconstructing a signal Y and for determining an error signal e.sub.y =Y-Y between the signals Y and Y.
These two channels are joined together with the aid of a phase comparator arrangement which receives the signals X, e.sub.x, Y, e.sub.y and a basic clock H (recovered symbol clock) and produces a comparator signal S, and also a variable-rate sampling clock which reproduces the basic clock H by excluding certain edges. The comparator signal S enters a bistable trigger under the control of said sampling clock. The bistable trigger output is connected to an amplifying filter which produces the oscillator control error signal .epsilon.(.phi.). The representation by in-phase and quadrature-phase signal points defines states of a signal constellation of said digitally modulated wave.
The invention also relates to arrangements in which the carrier recovery circuit is used such as digital transmission arrangements, more specifically those arrangements in which digital information, after having been transmitted by modulation of an electro-magnetic wave, is recovered by coherent demodulation. These arrangements are employed in data transmission modems, microwave radio links, space or optical communication systems for heterodyne links.
2. Prior Art
For effecting coherent demodulation, the phase of the carrier is generally recovered with the aid of an oscillator included in a carrier recovery loop. The voltage of this oscillator is controlled by a filtered version of the output signal of a phase comparator which detects the phase error between the oscillator and the transmission carrier.
For digital modulation schemes having a large number of states, the phase difference between the oscillator and the carrier must be limited to small values. The noise bandwidth of the recovery loop is then reduced by means of narrow-band low-pass filtering. This reduction results in very low phase noise, but also in a considerable decrease of acquisition range. To off-set this disadvantage, auxiliary arrangements are used for assisting in the acquisition.
In the prior art, amongst these auxiliary arrangements are frequency discriminators which generate a voltage depending on the frequency difference between the oscillator and the carrier, as described in, for example, French Patent Application No. 83 15794 (Publication No.: 2 552 959). This application relates to a carrier recovery circuit for digital modulation schemes having a phase symmetry 2.pi./M. In that circuit a variable-rate sampling of the output signal of the phase comparator changes the latter into a frequency discriminator. This property is obtained by eliminating zero-crossings of the phase comparator output which occur when the phase error between the oscillator and the carrier is equal to (.pi./M)+(2k.pi./M), where k and M are integers.
For multi-state digital amplitude modulation of two quadrature carriers, the frequency discriminator described in the above-mentioned application has drawbacks. Its gain is degraded, because at the sampled output of the phase comparator zero-crossings occur which are taken into account, but do not correspond to a zero phase error. Actually, starting from a basic clock H, this frequency discriminator generates a sampling clock, in which certain active edges of the basic clock H are eliminated. This elimination occurs when zero crossings are detected for which the phase difference between the oscillator and the carrier wave is equal to (.pi./M)+(2k.pi./M).
But experience has taught that, in phase and amplitude modulation, there are other zero-crossings at the output of the phase comparator which corrupt a correct acquisition of the carrier. In, for example, the case of 16 QAM modulation, the output of the currently used phase comparators have zero-crossings when the phase difference is equal to .theta.=arc tan (1/3) for the states (3, 1), (-1, 3), (-3, -1), (1, -3). Other states present a similar behaviour. In addition, in the case of a more important number of modulation states, the number of these unwanted zero-crossings will increase, and this causes a deterioration of the performances of the discriminator.